In typical 35 mm film cartridges, file filmstrip is withdrawn by its "film leader" from the cartridge and advanced past a rectangular exposure window or space gale and wound up on a take-up reel until all potential image frames of the filmstrip are exposed. In more recently proposed "thrust" cartridges, the filmstrip may be advanced out of the light trapping slit or mouth of the cartridge shell by rotating the film spool in an unwinding direction as described in commonly assigned U.S. Pat. No. 4,933,693, incorporated herein by reference in its entirety. Thus, in use, the cartridge may be loaded in the camera without any exposed leader protruding from the shell slit and advanced, after the camera film loading door is closed, by a motorized drive mechanism until the first image frame is positioned for exposure. The thrust cartridge avoids any unnecessary waste of the film leader and initial frames on loading into the camera. Moreover, it is contemplated that developed negative filmstrips may be returned to the customer wound back into the cartridge for safekeeping. In any case, the filmstrip is automatically or mechanically rewound into the film cartridge of either type upon exposure of all image frames, and the cartridge is removed for processing and printing or mounting, in the case of positive film.
Before the filmstrip is completely exposed, it may reside tightly wound in the cartridge or partially unwound within the camera for a period of time and under varying conditions of temperature and humidity such that the film takes a "core set" affecting the degree to which the film curls back upon itself when unrestrained. When the filmstrip is originally manufactured and cut to length, it is generally fairly limp and relatively easy to maintain straight (because it is wound on a relatively large diameter core and maintained at 50-60% relative humidity). After winding onto the core of the film cartridge, the innermost layers of the filmstrip wound around the core take on a higher degree of curl (given the relatively smaller diameter) than the outermost layers, and the degree of curl becomes more pronounced with the passage of time and exposure to elevated temperature and/or low humidity. Moreover, with use of the thrust cartridge to store developed film, core set film curl may become more likely and more pronounced with the passage of time and be more apparent when prints are re-ordered from the aged filmstrip. This is particularly true if the spool diameter is small and the film base is thin, relative to the thickness of the emulsion layers, in order to miniaturize dimensions of the cartridge.
Thus, the degree of core set film curl and its frequency of occurrence varies considerably due to its type, age, and environment to which it has been exposed in use and storage. This inconsistent core set film curl may make it difficult for photofinishing equipment to handle the film during splicing, processing and printing or mounting in certain photofinishing operations, individual filmstrips are withdrawn or advanced from the film cartridge and dragged through developing and fixing baths during processing, wherein the film transport mechanism does not restrain the free end of the filmstrip. A number of film base materials absorb fluids in the baths and become more limp than when they are first withdrawn from the cartridge. Other film base materials are more hydrophobic, and the excessive film curl, particularly of the more hydrophobic filmstrips, may cause the film to roll up or be folded back on itself and be damaged as it sticks together or may jam film transporting mechanisms in the processing, splicing, printing or mounting equipment. The film handling characteristics of transporting mechanisms vary widely and are typically preset or configured to accommodate an "average" film curl and are not readily adjusted without operator input. Thus, the equipment must be monitored, and halted if a malfunction occurs. Suspect filmstrips may be irreversibly damaged or have to be removed and be straightened, which may be difficult to do.
One solution to the problem would be to subject all filmstrips to a "de-curling" operation on withdrawal of the filmstrip from the cartridge and prior to processing, printing or mounting. Such de-curling operations include wrapping the filmstrip around a heated drum opposite to the curvature of the filmstrip for a certain time to straighten it. Doing so for every filmstrip would obviously introduce extra delay and cost and could negatively impact sensitometric performance. In this respect, if the applied heat and tension is not well controlled and tailored to the film type, more sensitive film types may shift in color balance or speed resulting in lower quality prints. The additional handling itself introduces the possibility of physical damage, e.g. scratching of the emulsion layers.
It would be therefor be desirable to overcome the problems and risks in film handling occasioned by the infrequently occurring excessively curled filmstrip without subjecting all filmstrips to de-curling.